The study is being performed within the Pediatric Brain Tumor Consortium, a group of institutions nationwide, that specialize in treating children with brain tumors. Two groups of pediatric CNS tumor patients are eligible-those with diffuse intrinsic pontine glioma which is a deadly disease which primarily strikes children 5-10 years of age and has a median survival of less than 1 year, and recurrent high-grade gliomas which also has a similarly poor prognosis. The study is being performed in two stages. An initial first stage is performed in 16 patients for each cohort; if an objective response is observed, the cohort will expand (second stage). If no responses are observed, the study for that arm will be closed. Patients are treated for 5 days of every 28 day course with IV O6BG at a dose of 120 mg/m2/d x 5 days administered over 1 hour. Temozolomide is given orally after completion of the infusion at a dose of 75 mg/m2/d x 5, the dose identified as the maximum tolerated dose in our Phase I study. We have reached our accrual goals for this study and patients are being followed until the endpoint is reached. We are also reviewing available pathological tissue obtained from patients at initial diagnosis (i.e. not obtained as part of this study) to verify pathologic diagnosis in patients who have long term stable disease or partial responses. This study is now closed to accrual and undergoing analysis.